1. Field of the Invention
The present invention relates to a wafer-polishing head which is used in an apparatus for polishing the surface of wafers such as a semiconductor wafer or the like, and a polishing apparatus having the same.
2. Description of the Related Art
Recently, fineness of patterning of wafers has become important, in accordance with high level of integration thereof. It has become especially important to flatten semiconductor wafers where the patterning is formed and to flatten the surface of semiconductor wafers as much as possible in the process of forming the patterning, such that the formation of a fine pattern having a multi-layer structure can be carried out easily and surely. For example, the formation of a pattern is carried out using optical lithography, and the depth of focus in the optical lithography becomes shallow in proportion to the fineness of a pattern. In order to increase the accuracy of a pattern and to easily control focus during exposure, it is required that a difference of unevenness on a wafer surface is no greater than the depth of focus (that is, flatness).
Further, with regard to polishing bare wafers, a requirement for flatness has become severe in proportion to the increased size of the wafer. In this case, a chemical and mechanical polishing method (CPM method) is known which provides a high flatness to the polished surface of semiconductor wafers (referred to as a wafer, hereunder). In the CMP method, the surface of the wafer is polished chemically and mechanically to obtain a flatness of the surface using an alkaline slurry containing SiO2, a neutral slurry containing CeO2, an acid slurry containing Al2O3 or a slurry containing abrasive grains (hereunder, these slurries are generally referred to as abrasive grain-containing slurries). For example, a polishing apparatus shown in FIG. 4 is known for polishing the surface of wafers according to the CPM method.
As schematically shown in FIG. 4, which is an enlarged perspective view of an important part of the CPM, a polishing pad 104 comprising, for example, hard urethane, is provided on a disc-shaped platen 103 attached to a center shaft 102 in a polishing apparatus 100, and a wafer-polishing head 105 which holds a wafer W is disposed facing the polishing pad 104 and in a position eccentric to the center shaft 102 of the polishing pad 104 such that the polishing head can be rotated on its own axis. In the polishing apparatus 100, the polishing pad 104 and the wafer W are moved relatively to each other while the wafer W is pressed against the surface of the polishing pad 104 by the wafer-polishing head 105. The above-mentioned abrasive grain-containing slurry S is interposed between the wafer W and the polishing pad 104, thereby to polish one surface of the wafer W. The polishing apparatus 100 as shown in FIG. 4 is only one example. There are known polishing apparatuses having a plurality of wafer-polishing heads 105.
Next, one example of wafer-polishing pad 105 is schematically shown in front sectional view in FIG. 5(a). The wafer-polishing head 105a shown in FIG. 5(a) comprises a principal head 106 having a lower opening portion 106a and having an approximately circular shape. A flexible membrane 108 is positioned inside the principal head 106 for closing the opening portion 106a and for holding a wafer W at the underside of an inward peripheral portion 108a. A sealed pressurizing chamber 107 is formed by closing the opening portion 106a with the flexible membrane 108. A pressure regulator 109 is connected to the pressurizing chamber 107 to control the internal pressure thereof.
The flexible membrane 108 comprises a flexible film and is fixed to the principal head 106 so as to be approximately planar over the full face thereof, for example by putting the outer peripheral portion of the flexible membrane between the principal head 106 and a guide ring 110 which has an approximately annular shape and is attached to the end of the opening portion of the principal head 106, as shown in FIG. 5(b) which is a partially enlarged view of FIG. 5(a).
The pressure regulator is used for adjusting a force for displacing the flexible membrane 108 in the axial direction of the head by controlling the internal pressure of the pressurizing chamber 107. In the wafer-polishing head 105a, a force for displacing the flexible membrane 108 in the axial direction of the head, that is a force by which the flexible membrane 108 presses a wafer W against a polishing pad 104 (polishing pressure), is adjusted such that the polishing pressure for the wafer W is controlled within a proper range thereof.
A guide ring 110 is attached to the principal head 106, surrounding the end of the opening portion of the principal head. When the wafer W is polished, the guide ring protects the wafer W from dropping out from the wafer-polishing head 105a while it is pressed against the polishing pad 104, and decreases the deformation of the polishing pad 104 by pressing the polishing pad in a portion surrounding the wafer W, with the result that the working accuracy at the outer peripheral edge of the wafer W is increased.
A wafer-polishing head having a structure mentioned above is disclosed in JP-A 5-69310, for example.
Other wafer-polishing heads are also known. FIG. 6(a) is a front sectional view schematically illustrating a wafer-polishing head 105b and FIG. 6(b) is a partially enlarged view of FIG. 6(a). The wafer-polishing head 105b of these figures comprises a principal head 111 having a lower opening portion 111a and having an approximately circular shape; a holding member 112 is disposed inside of the principal head 111; a flexible membrane 114 having an outer peripheral portion 114b is held by the holding member 112 so that the outer peripheral portion 114b of the membrane is turned up and inward toward the peripheral portion 114a; and a pressurizing chamber 113 is formed inside the opening portion 111a by the flexible membrane 114. Here, a pressure regulator 115 is connected to the pressurizing chamber 113 to control the internal pressure thereof. Further, in this example a retaining ring 116 having an approximately annular shape is provided in the lower end of the wafer-polishing head 105b, surrounding the periphery of the opening portion 111a. The flexible membrane comprises a flexible film and holds a wafer W underneath the lower surface thereof. In the same manner as in the wafer-polishing head 105a, a force by which the flexible membrane 114 presses the wafer W against a polishing pad 104 (polishing pressure) is adjusted by controlling the internal pressure of the pressurizing chamber 113 with the pressure regulator 115. The retaining ring 116 has almost the same function as the guide ring 110 of the wafer-polishing head 105a. 
A wafer-polishing head having the structure mentioned above is disclosed in JP-A 10-337658, for example.
The above-mentioned wafer-polishing heads 105a and 105b apply polishing pressure to the wafer W by transferring the internal pressure of the pressurizing chamber to the wafer W thorough the flexible membrane, and so the membrane is required to be as thin as possible so that the elasticity of the flexible membrane has no influence on the polishing pressure to the wafer W.
However, as the flexible membrane is apt to deform due to its thinness, it can enter a gap formed between the peripheral end of the wafer W and the inward peripheral portion of the guide ring 110 (or the inward peripheral portion of the retaining ring 116) when the wafer W is pressed against the polishing pad by the internal pressure of the pressurizing chamber. See FIGS. 7(a) and 7(b). In this case, as the underside of the membrane is not pressed against other members such as the wafer W and the guide ring 110 (or the retaining ring 116), and since the membrane receives the internal pressure of the pressurizing chamber, the deformed part is subject to a heavy load as compared with the other parts. In addition, the deformed part is interposed between the wafer W and the guide ring 110 (or the retaining ring 116), with the result that it can suffer damage. The deformation of the flexible membrane reduces its useful life.
Further, as shown in FIG. 7(b), in the wafer-polishing head 105b, the outer peripheral portion 114b of the flexible membrane 114 is held by the holding member 112 such that the outer peripheral portion 114b of the membrane is turned up and in the inward direction. As a result, when the internal pressure of the pressurizing chamber 113 is applied to the inward peripheral portion 114a of the flexible membrane 114, the turned up part of the membrane is stretched and is subject to a heavy load as compared with the other parts. In addition, as the turned up part is pressed against a corner where the outer peripheral face and the under surface of the holding member 112 meet, a heavier load concentrates on the turned up part.
Furthermore, the following phenomenon is found in both of the wafer-polishing heads 105a and 105b. The case of the wafer-polishing head 105a is described hereunder, referring to FIG. 7(a). The outer peripheral edge of the wafer W is usually beveled. As a result, when the wafer W is pressed against the polishing pad by increasing the internal pressure of the pressurizing chamber, the flexible membrane 108 goes around the beveled outer peripheral edge and a surface area where the wafer W contacts the flexible membrane 108 is larger than that where the wafer contacts the polishing pad 104. The difference between two areas is shown at d in FIG. 7(a).
Therefore, a magnitude of the pressure on the polishing pad 104 (polishing pressure) is greater in the outer peripheral edge of the wafer W than in the other parts thereof. As a result, more material is removed in the outer peripheral edge of the wafer W than in the other parts thereof, which has made it difficult to attain a high polishing accuracy of the wafer W (a uniform amount of polishing of wafer). In FIG. 7(a), a direction of the force applied to the wafer W and flexible membrane 108 is indicated by arrows, and a magnitude of the corresponding force is indicated by the length of the arrows.
Still farther, in case of polishing a wafer using the above-mentioned wafer-polishing heads, because it is more difficult for the abrasive grain-containing slurry to enter the inward peripheral portion of the wafer W than the outer peripheral portion thereof, an amount of polishing is generally apt to be greater in the outer peripheral portion than in the inward peripheral portion. Therefore, it is required to restrain an amount of polishing of the outward peripheral portion of the wafer.
The present invention has been made, considering the above-mentioned situation. An object of the present invention is to provide a wafer-polishing head which permits a longer life of the polishing apparatus, and a greater polishing accuracy for wafer.
According to a first aspect of the present invention, the above and other objects are addressed by a wafer-polishing head which polishes a wafer while the wafer is pressed against the surface of a polishing pad attached on a platen and is moved relatively to the polishing pad. The wafer-polishing head comprises a principal head having an opening portion in the lower portion and having an approximately circular shape; a flexible membrane having an approximately circular shape which closes the opening portion and holds the wafer at an inward peripheral portion thereof, a pressurizing chamber formed by the flexible membrane; wherein the flexible membrane is thicker in the outer peripheral portion thereof than in the inward peripheral portion which holds the wafer.
In the wafer-polishing head having a construction mentioned above, as the flexible membrane is thicker in the outer peripheral portion than in the inward peripheral portion which holds the wafer, adequate strength can be ensured in the outer peripheral portion which is susceptible to a load, while restricting to a low value an elastic force in the portion of the flexible membrane which holds the wafer.
Further, as the thickness of the outer peripheral portion is greater, the flexible membrane is hard to deform in the vicinity of the outer peripheral edge of the wafer.
The present invention is applicable to a flexible membrane which is positioned in the opening portion of a principal head with the outer peripheral portion of the membrane turned up and inward, as well as a flexible membrane which is approximately planar throughout.
According to a second aspect of the present invention, the above and other objects are addressed by a wafer-polishing head wherein a thickness of said flexible membrane tapers in such a manner that a portion of the membrane which holds the vicinity of the outer peripheral edge of the wafer is thicker in an outward direction.
In the wafer-polishing head having a construction mentioned above, since a portion of the membrane which holds the outer peripheral edge of the wafer is tapered to be thicker at the outer peripheral side, the thicker portion is harder to deform. Thereby, in the portion which holds the outer peripheral edge of the wafer, a force by which the wafer is pressed against a polishing pad is decreased towards the outer peripheral side when the internal pressure of a pressurizing chamber is raised, with the result that an amount of polishing the wafer changes gently in the corresponding portion.
Further, since a change of thickness is gentle in the boundary between the thin inward peripheral portion and the thicker outer peripheral portion, a load will not concentrate at one portion when a pressure is applied to the flexible membrane from the pressurizing chamber.
According to a third aspect of the present invention, the above and other objects are addressed by a wafer-polishing head, wherein the flexible membrane is positioned in the opening portion of the principal head with the outer peripheral portion of the flexible membrane turned up and inward, and the outer peripheral portion of the flexible membrane is disposed above the inward peripheral portion thereof, where said outer peripheral portion of the flexible membrane comprises at least a portion facing the outer peripheral edge of the wafer and another portion disposed outward from the portion facing the outer peripheral edge of the wafer.
In the wafer-polishing head having a construction mentioned above, as the flexible membrane is positioned in the opening portion of the principal head with a portion of the flexible membrane facing the outer peripheral edge of the wafer and another portion of the flexible membrane disposed outward of the portion facing the outer peripheral edge of the wafer (the two portions are together designated as the outer peripheral portion), and the outer peripheral portion of the flexible membrane is disposed above the inward peripheral portion thereof, the flexible membrane is pressed against a portion of the wafer which is limited to a range within the inward peripheral side of the wafer from the outer peripheral edge of the wafer. Thereby, the contacting area of the wafer and the flexible membrane is smaller than that of the wafer and the polishing pad, with the result that a force by which the wafer is pressed against the polishing pad (polishing pressure) in the outer peripheral edge of the wafer is restrained.
According to a fourth aspect of the present invention, the above and other objects are addressed by a polishing apparatus for polishing a wafer comprising a platen having a polishing pad attached thereon; a wafer-polishing head which holds one face of the wafer to be polished while another face of the wafer is pressed against the polishing pad; wherein polishing of the wafer is carried out by providing relative movement of the platen and the wafer while the wafer is pressed against the polishing pad.
In the polishing apparatus having a construction mentioned above, wafers can be polished by using functions and effects provided by the wafer-polishing head according to any one of the first to third aspects of the present invention.